Building the World’s Longest — And Smartest — Floating Bridge

Fighting the Wind

In the bridge’s current form, high winds send Lake Washington waves crashing across the roadway, prompting officials to close the floating bridge in adverse weather. By elevating the bridge deck 20 feet above the water level, weather-related events are all but eliminated. And it also takes care of maintenance concerns.

The new bridge design means crews won’t have to travel on the road to perform maintenance. Instead, they’ll access the pontoons through dedicated spaces under the roadway. If they need to get up to the tarmac, stairways within the bridge supports provide additional access, but that won’t be happening very often – 98 percent of maintenance access to the bridge will now be done via boat and away from traffic.

Cheating Mother Nature

A custom-developed weather station located on the bridge will monitor wind speed and direction, and will be a massive improvement over the existing systems.

The current bridge system trips an alarm if sustained gusts of 40 mph last for a minute. Because of the new aerodynamic design, the wind criteria will be increased for the new bridge and the system will still tie into the PLC to alert crews to keep an eye on sway. However, Allen says, “it will take a real whopper to shut [things] down.” Additionally, sensor-laden “pucks” embedded in the roadway will transmit deck temperature information to crews, alerting them of possible sand or if it’s time to bust out the de-icing equipment.

Quiet Concrete and Smart Sensors

For the first time in a project of this scale, mixed-generation concrete – or what’s also known as “quiet concrete” – will cover the surface of the bridge. By using advanced grinding methods, the particles of the concrete mixture reduce the noise of vehicles traveling on pavement, which is particularly important for neighbors living on the shores of sound-reflecting water.

The roadway will also contain traffic data monitoring equipment to get speed and the amount of vehicles traveling on the roadway, along with active traffic management signage. Bridge officials hope to add a high-efficiency street sweeper to help remove debris and contaminants before it goes through the bridge’s stormwater system.

Improved Lighting

Old-school streetlights will give way to LED and fluorescent lights. The iconic “sentinels” on each end of the bridge use LEDs to change the color and style of lighting. The translucent spires atop each sentinel house additional lights, illuminating the structure from the inside. Fluorescents handle roadway illumination, while metal-halide lighting will keep the 10-foot pathways for pedestrians and cyclists illuminated at night.

The world's longest floating bridge, a 230,000-ton ribbon of concrete spanning Lake Washington, is getting longer. And smarter. The Washington State Department of Transportation is rebuilding State Route 520 that links Seattle to all points east, making it safer and more efficient for the Evergreen State's drivers.

But it’s not the new bridge’s 7,710 feet of tarmac, 77 concrete pontoons, 58 reinforced concrete blocks or the 3-inch-thick cables keeping it together that’s impressive. After all, basic physics keep it afloat – the weight of the structure equals the weight of the water being displaced. No, advanced construction methods and new technologies, including electrified rebar and hundreds of moisture sensors, will play prominent roles in building the 116-foot-wide, 20-foot-high, six-lane structure, while helping the bridge retain its “world’s longest” title.

How to Pour a (Massive) Pontoon

The new SR 520’s pontoons are 360 feet long, 75 feet wide and 28 feet tall (although only six feet are visible above the water), and weigh the equivalent of to 23 Boeing 747 jets. But it’s not the tonnage that’s an issue.

The sheer volume of the concrete being poured causes a massive amount of heat to build up. So the team responsible for the construction had to get creative, installing tubes filled with cooling liquid (think your car’s radiator, but on an epically large scale) to keep temperatures in check. “If it gets too hot, it can lose strength and lead to cracking and brittleness,” says George Fies, Washington State Department of Transportation’s (WSDOT) floating-bridge engineer. Before floating pontoons out to Lake Washington where they must deal with the elements, any "structural crack" over 6/1000th of an inch gets fixed with a epoxy injection and crystalline waterproofing, while smaller cracks need just a crystalline treatment.

Keeping Things Afloat

Over 1,000 water sensors located within the pontoon structures connect remotely to the bridge’s main Programmable Logic Controller (PLC). As soon as a leak or any other anomaly is detected, an alert goes off and maintenance personnel are sent to investigate.

Within each pontoon-mounted, watertight cell sits a float switch, located a scant three inches off the pontoon’s floor. If rising water triggers the switch, the sensor will send a signal to a central monitoring panel, then relay the information to the PLC at the east end of the bridge. From there, the system will instruct an auto dialer to call 24-hour dispatch. “This is the first time this has been done at this scale,” says Archie Allen, WSDOT bridge superintendent. And obviously, they’re hoping it never goes off.

Electrifying Rebar

With steel rebar inserted throughout the concrete pontoons and steel cables connecting the bridge to anchors, corrosion is inevitable, and that’s where a “cathodic protection system” comes into play.

A system keeps tabs on corrosion and a cathodic protector automatically pumps low-voltage, DC electricity into the metal to counteract the effects of the water. The bridge features one system focused solely on the pontoons and another monitoring the cables. “To start out with, [the voltage] should be very little, but it will keep electrolysis in check,” Allen says.

Maintenance Finally Has a Home

Between eight and 10 maintenance personnel will be available 24 hours a day, all housed in a LEED Silver-certified building at the east approach of the bridge.

Along with a yard for equipment and offices, the facility will also contain the command station with monitors for viewing security cameras, scads of computers, a primary and backup server, along with the rest of the PLC system. Additionally, the facility has an energy-monitoring system, a backup power source, new docks for the maintenance boat and a hydroponic system to defrost the dock. Intrusion alarms on each pontoon will also alert the maintenance facility and prompt a call-out – one of 15 possible reasons the red phone will ring.

Fighting the Flames

“I wouldn’t call it fire-suppression, I would call it a fire-fighting system,” Allen notes when describing the automated system that will rise from the center of the bridge to douse vehicle fires.

With four 350-horsepower electrical pumps (two at each end of the bridge for redundancy) supporting almost 8,000 lineal feet of fire pipe, the bridge can stock hydrants every 900 feet with lake water. Once activated from the command room, the system can charge either the west or east sections in about eight minutes. When finished, the lines automatically drain. By using lake water, engineers didn’t have to deal with the issue of running water lines through the highly movable transition point between the fixed bridge and floating bridge, allowing the placement of hydrants on a floating bridge for the first time.

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